Formation of Chlorine in the Atmosphere by Reaction of Hypochlorous Acid with Seawater

The highly reactive dihalogens play a significant role in the oxidative chemistry of the troposphere. One of the main reservoirs of these halogens is hypohalous acids, HOX, which produce dihalogens in the presence of halides (Y–), where X, Y = Cl, Br, I. These reactions occur in and on aerosol particles and seawater surfaces and have been studied experimentally and by field observations. However, the mechanisms of these atmospheric reactions are still unknown. Here, we establish the atomistic mechanism of HOCl + Cl– → Cl2 + OH– at the surface of the water slab by performing ab initio molecular dynamics (AIMD) simulations. Main findings are (1) This reaction proceeds by halogen-bonded complexes of (HOCl)···(Cl–)aq surrounded with the neighboring water molecules. (2) The halogen bonded (HOCl)···(Cl–)aq complexes undergo charge transfer from Cl– to OH– to form transient Cl2 at neutral pH. (3) The addition of a proton to one proximal water greatly facilitates the Cl2 formation, which explains the enhanced rate at low pH.


Authors reply and revisions made:
We appreciate the reviewer's concern.In the manuscript we have stated that "Sea salt aerosols originate at the same pH as seawater (~8) but, within minutes, undergo a pH drop of a few pH units" which indicated that seawater pH is alkaline.However, there are many conditions where locally sea water is acidic e.g., the aerosol generated from the sea water is acidic.Moreover, recent reports show substantial fraction of emitted CO2 emissions absorption by the oceans is resulting in a reduction of seawater pH 1,2 which would lead to reduce the seawater pH towards acidic, leading interfacial Cl2 formation.In the revised manuscript, we added the sentence "However, the value of pH ~8 is an average value and there are many conditions where locally sea water is acidic" to clarify the reviewer's concern (highlighted in yellow).

Comment 2:
It is mentioned that all H atoms have been substituted by D atoms at the crucial part of the simulation to suppress quantum mechanical tunnelling.First, this should also be said clearly in the main text.Second, this idea does not help with the quantum mechanical zero-point energy which can be different for reactants and products.This could be elaborated a bit in the main text.

Authors reply and revisions made:
We thank the reviewer to point out the valuable point.In the revised version of the manuscript, we have added "We substitute hydrogen atoms by deuterium to accommodate a larger time step for the simulations" (highlighted in yellow) in the Section "Stability of (HOCl)…(Cl -)aq complexes in water".We agree also with the second point of the reviewer, about the zero-point energy difference of the reactants and products.However, at 300K (temperature at which we simulated our systems) the role of zero-point energy is expected to be small.Therefore, we neglected the effect in the present study.

Minor points:
Response: We are grateful to the reviewer to carefully spot the typographical issues.1) All non SI-units (Å, kcal/mol, Ry) should be defined as SI units first time these appear in the manuscript.

Authors reply and revisions made:
We change a H-Cl -→ an H-Cl -and an Cl-Cl -→ a Cl-Cl -on Page 6.

Authors reply and revisions made:
We modified Data is to Data are in Figure 3.

Summary:
The activation (oxidation) of halides to reactive forms is of great importance in atmospheric chemistry.In spite of many experimental studies, both in the lab and in the field, real questions remain concerning the molecular mechanisms involved.In this work the authors describe ab-initio MD studies of the reaction of HOCl with Cl -at (or near to) an air-water interface.They present arguments that the reaction proceeds via a halogen-bonded complex, and that the presence of a hydronium ion in the solvation shell promotes reaction significantly.
Response: We thank the reviewer for his/her encouraging remarks.
Comment 1: While both of these findings are really interesting the really novel aspect of the work to me is the existence of a long-lived halogen-bonded complex, and its importance to the reaction.That this is non-intuitive is hinted by the authors, but the physical reason(s) are not addressed.Indeed, the authors statte that the halogen-bonded complex is less strongly bound than the hydrogen bonded one, yet both are (equally) long-lived!! Please -some physical insights here about how this happens!I assume that solvation plays some role; understanding exactly what role will surely give greater insight into this reaction.

Authors reply and revisions made:
We regret the lack of clarity about the physical insights of the stability of halogen bonded complexes.As the reviewer pointed out correctly, both hydrogen and halogen bonded complexes are (equally) long lived during our simulation time of 20 ps.We clarify that these complexes are stable at least 20 ps which is enough to act as pre-reactive complexes.The exact lifetimes of these complexes are not known as we run simulation only up to 20 ps, longer simulations would be costly due to system size and not relevant of the problem addressed in the paper.The reason of stabilization of halogen bonded complexes in water is the water envelop surrounding the complexes provides additional binding stabilization.In the case of hydrogen bonded complexes, the hydrogen bond partner of HOCl has competition between Cl -ion and other solvent water molecules.We added in the revised manuscript the sentence "This stabilized water envelop provides enough lifetime to the halogen bonded complexes" (highlighted in yellow) on Page 6 to clarify this point.
Comment 2: I also wonder about the authors' thoughts on the role of entropy in driving the reaction.The acid-promoted reaction has water as a product; in solution this would not necessarily be favored to the same extent as at an interface.So the mechanism *may* be somewhat interface-specific -do the authors have thoughts on this?Authors reply and revisions made: We agree with the reviewer's comment.In fact, because of the entropy effect, the reaction is expected to be enhanced at the interface with respect to bulk.However, we did not quantify the effect.We have referred about this in the on Page 12 (highlighted in yellow).Moreover, we tried to put the complex inside the water slab (Data not shown), but after 3 ps the complex reaches the interface mentioned in the Computational Methods Section.As the pre-reactive complexes are only stabilized on the interface, the mechanism is interface specific.

Response to the Editor's comments:
We modified References 1-55 in the main text (highlighted in yellow) according to the JPCL format in the revised manuscript.We included the page numbers in the SI.References: (1) Ríos, A.